As patients live longer and are diagnosed with chronic and often debilitating ailments, there is an increased need for improvements to the speed, convenience, and efficacy of drug delivery. For example, many chronic conditions, including multiple sclerosis, diabetes, osteoporosis, and Alzheimer's disease, are incurable and difficult to treat with currently available therapies: oral medications have systemic side effects; injections may require a medical visit, can be painful, and risk infection; and sustained-release implants must typically be removed after their supply is exhausted, and offer limited ability to change the dose in response to the clinical picture. In recent decades, several types of wearable drug delivery devices have been developed, including battery-powered miniature pumps, implantable drug dispensers, and diffusion-mediated skin patches.
Treatments for a number of chronic diseases currently require subcutaneous administration of a drug or therapeutic agent either continuously or at specific times or time intervals in highly controlled doses. Subcutaneous injections take advantage of the lack of blood flow to the subcutaneous layer, which allows the administered drug to be absorbed more slowly over a longer period of time (compared with direct injection into the bloodstream). Additional advantages to subcutaneous delivery of some drugs (i.e., vaccines, tuberculin tests, immunostimulants, etc.) to the tissue region are the targeting of lymph tissue and lymphatic drainage for subsequent antigen presentation to the body. Traditionally, these types of injections have been administered either by the patient or a medical practitioner anywhere from several times a day to once every few weeks. Such frequent injections can result in discomfort, pain, and inconvenience to the patient. Self-administration further poses the risk of non-compliance or errors in dosage events.
These problems can be at least partially overcome by wearable, electronically controlled drug pump devices capable of delivering highly controlled dosages of drug continuously or intermittently, depending on the needs of the patient. Such pump devices may be programmed to deliver drug in accordance with specified delivery protocols for extended periods of time, which can, in some circumstances, virtually eliminate the need for human control of pump operation. Of course, once the drug reservoir of a pump device is empty, human intervention is necessary to replace or refill the device. To avoid unexpected depletion of the drug reservoir and minimize the interruption of drug delivery to the patient, it would therefore be desirable to equip the pump device with features for detecting the filling status of the reservoir and alerting the patient or clinician when the drug is nearly used up.